The present invention relates to a high voltage DC switchyard comprising                at least one busbar,        at least two DC lines each connected to said at least one busbar through DC breakers comprising a section of at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith,        means configured to detect occurrence of a fault current in connection with said busbar or DC lines, and        a control unit configured to control said DC breakers for protecting equipment connected to said busbar and/or DC lines upon occurrence of a said fault current.        
High voltage means a voltage ≧10 kV and often a voltage of several hundreds kV with respect to ground.
Such a DC switchyard is used for interconnecting high voltage DC lines. This may for instance be done for forming a DC transmission network or for just interconnecting long High Voltage Direct Current transmission lines, which may be overhead lines and/or cables. AC/DC converters may be included in the switchyard and have a DC side thereof connected to said busbars for also connecting an alternating voltage network or a generator of electric power to the DC switchyard. The number of DC lines connected to such a DC switchyard may be any conceivable, although two and four are shown in the Figures of this disclosure.
One possible configuration of such a DC switchyard in the form of a one and a half breaker switchyard is schematically illustrated in appended FIG. 1, in which three DC breakers 1-3 and 4-6 are arranged per two DC lines 7, 8 and 9, 10, respectively, for connecting these DC lines to the two busbars 11, 12 of the switchyard.
Another possible configuration of such a switchyard in the form of a so-called two breaker switchyard also having two busbars is schematically illustrated in appended FIG. 2. This switchyard has each DC line 20, 21 connected through two individual DC breakers 22, 23 and 24, 25, respectively, to a redundant system of busbars 26, 27.
The DC breakers of such switchyards are arranged for breaking fault currents occurring as a consequence of faults within the switchyard or in the DC lines connected thereto for isolating the fault and protecting other parts of the power transmission system. It is then of great importance to be able to at a very short notice, such as in the order of a few 100 μs, upon occurrence of such a fault limit this fault current for preventing severe impacts upon equipment connected to the system, which is the reason for using semiconductor devices of turn-off type, which may open within a few μs, as switches in such DC breakers.
However, such semiconductor devices of turn-off type, such as IGBT:s (Insulated Gate Bipolar Transistor), of standard design are not able to block negative voltages. A bidirectional such DC breaker based on standard IGBT:s without reverse blocking capability is shown in FIG. 3 and is composed of two said sections each including an IGBT 30, 31 and an anti-parallel diode 32, 33, which are connected in series with opposite current direction. It is noticed that for each current direction the current will pass through one IGBT and one diode, which will result in conduction losses in the IGBT and in the diode, in which the losses in the diode will be approximately 50% to 70% compared to the on-state losses in the IGBT. Furthermore, a large number of series connected bidirectional IGBT current switches shown in FIG. 3 is required to form a bidirectional high voltage DC breaker together with a parallel connected common arrester bank for current limitation not shown in the Figure.